The present invention is directed to pressure sensitive hot melt adhesives used as lining adhesives in the production of hard bound books.
"Hard bound" books conventionally consist of two main components: the case and the body. The case comprises the front and back covers connected by the spine. The body of the book, also known as the "block", is comprised of a plurality of paper sheets or "leaves" which are generally sewn together along their center folds in groups to form the sections or "signatures" of the book. The signatures are then assembled to form the body of the book. At the front and back of the body are affixed heavy folded pages, "end papers", which facilitate the attachment or "casing-in" of the body to the case.
Alternatively, the book block may be formed using a "perfect binding" technique. In this operation, the pages are printed in long, continuous sheets or webs which are cut, folded and properly arranged to form a series of stacks, referred to as signatures, which are then fed into a continuous, revolving chain equipped with clamps. The free edges of the clamped sheets are first cut to remove the signature folds and then roughed so as to expose a maximum amount of fibers at the edges of the sheets and to make them more receptive to the adhesive composition. A layer or coat of a hot melt adhesive composition is then applied in molten form by means of a suitable applicator, after which the end papers are applied before the molten adhesive sets to form the spine of the book. A third procedure for the production of the book block involves the burst, perforate or notched method where the book block maintains the signature fold but contains burst, perforations or notches into the innermost page of the signature and wherein the adhesive is applied to the burst, perforate or notched signature to form the book block.
After the block is formed, the next step in the manufacture of books is rounding and backing if the book is intended to have a curved or rounded configuration. After the rounding, a reinforcing material called a "super" or "crash" is applied to the spine. This is done by gluing the center of the guaze-like scrim "super" or "crash" to the book block or spine. The ends of the scrim extend about 7/8 inch along the outside of the end papers on either side of the body. After the scrim or super is applied, a second coat of adhesive is typically applied and a kraft strip, with optional headband, is applied. In some operations, sufficient adhesive is applied in the gluing of the scrim to penetrate the scrim and, in these cases, the second adhesive coating step is not required. The kraft backing applied in registered relation to the book spine, the optional headbands and the scrim combine to form the conventional lining for the book.
The lined book block is then inserted into the case so the folds in the end papers are adjacent the spine of the case. Adhesive is applied to the front and back end papers adjacent the inside of the covers of the case. The end papers are pasted to the inside of the covers, enclosing the crash. The case and body are then passed through a "building in" operation to form the groove at the either edge of the case spine or placed in a press. When the attachment process, termed "casing-in" is complete, the "body" is bound to the cover incorporating a hinge so that the covers and leaves are movable with respect to each other.
Since there is no direct attachment of the book spine to the cover, it will be recognized that the lining operation and the materials used therein are critical to the production of a satisfactory book. Specifically, the adhesive(s) used to attach the scrim and kraft liners must be sufficiently low in viscosity (e.g. 500 to 5000 cps at 350.degree. F.) to facilitate application; they must not bleed through the kraft; they must exhibit superior adhesive and cohesive strength; the films formed therefrom must be flexible to allow for easy opening even at low temperatures so as to resist "cold cracks" and yet must not flow or creep at room temperature or at elevated temperatures.
Non-pressure sensitive hot melt adhesives have recently replaced animal glues in these applications but these adhesives are deficient with respect to their open time. This lack of open time creates a serious problem in the automated book binding process where interruptions, slow downs and even shut downs of the manufacturing line result in loss of agressive tack properties with the subsequent need to discard large quantities of partially completed products. Another problem consequent with many of the hot melt adhesives presently used for this application is their high level of rigidity when solidified, resulting in a stiff backbone which is not desired for an "easy opening" book.
It is therefore a object of the present invention to provide an adhesive composition for lining hard bound books which has the cohesive strength, temperature resistance, etc. of previous adhesives without being limited with respect to loss of agressive tack over time.
It is further object of the invention to provide an adhesive composition for lining hard bound books which remains flexible so as to produce an easy opening book.
These and other objects will be apparent from the description that follows.